px:html-load support for non-UTF8 encodings

The HTML loading is changed as follows:

 * The HTML is first loaded using UTF8 as a default encoding.
 * the real encoding declared in the `Content-Type` metadata is parsed
   from the loaded XML
 * if the encoded differs, the file is loaded again using the real
   encoding.

Note: the character encoding info is extracted following the [HTML5 rules](http://www.w3.org/TR/html5/infrastructure.html#extracting-character-encodings-from-meta-elements)